Alkadienols, especially, octadienol is a chemical industrially important compound as an intermediate for producing n-octanol and an ester or the like thereof.
As a method of producing alkadienols, a method of reacting a conjugated alkadiene with water in the presence of a catalyst composed of a palladium compound and a phosphine or a phosphite, and carbon dioxide is conventionally known.
For example, Japanese Patent Application Laid-Open (KOKAI) No. 54-144306 discloses a process for producing 2,7-octadiene-1-ol comprising a hydration and dimerization step for producing octadienol by reacting 1,3-butadiene with water in the presence of a catalyst composed of a nonaqueous palladium compound and a phosphine or a phosphite, carbon dioxide and an organic solvent; a low-boiling point component separation step for separating a low-boiling point by-product, water and the organic solvent from the liquid reaction product (reaction mixture) obtained in the hydration and dimerization step so as to obtain a component containing octadienol, a high-boiling point by-product and the catalyst; a flash distillation step for flash-distilling the obtained component containing octadienol, the high-boiling point by-product and the catalyst so as to separate a fraction containing octadienol from a high-boiling point component containing the high-boiling point by-product and the catalyst; a catalyst recovery step for separating the high-boiling point by-product from the high-boiling point component so as to obtain a catalyst solution; and an octadienol refining step for distilling the obtained fraction containing octadienol so as to separate 1,7-octadiene-3-ol and a high-boiling point ingredient, thereby refining 2,7-octadiene-1-ol.
It is described that the low-boiling point by-product separation step in the above-mentioned process is carried out at a temperature of not higher than 110.degree. C. in order to remove 1,3,7-octadiene which is a by-product and the low-boiling point solvent such as acetone, from the reaction mixture. It is also described that a remaining liquid in the distillation still is composed of an organic phase and an aqueous phase, and that the aqueous phase separated from the remaining liquid is recycled to the hydration and demerization step.
In the low-boiling point component separation step for separating a low-boiling point component from the reaction mixture, if the distilling temperature is too high, for example, if it exceeds 120.degree. C. the alkadienols produced is decomposed and/or polymerized so that the yield is reduced, and the palladium catalyst is metallized. Such a loss in the alkadienols and the palladium catalyst is not only caused by the high distillation temperature but also influenced by the method of supplying the heat which is necessary for the distillation.
The heat necessary for distillation is generally indirectly supplied to a bottom liquid in the distillation column through a heat exchanger such as a shell-type kettle, a tubular-type kettle and a jacketed-type kettle. In such a heat supply system (indirect heating system), since the bottom liquid is heated by passing a heating medium heated to a temperature generally at least 30.degree. C. higher than the temperature of the bottom liquid, through the heat exchanger, the temperature of the heating surface of the heat exchanger which is in contact with the bottom liquid in the distillation column is higher than the temperature of the bottom liquid. For example, the temperature of the transfer surface becomes not lower than 120.degree. C. Therefore, in the case of the indirect heating, it is difficult to prevent the decomposition and/or polymerization of the alkadienols, and the metallization of the palladium catalyst due to the local heating by the heating surface of the heat exchanger, so that the loss in the alkadienols and the palladium catalyst is very large.
That is, although the method disclosed in Japanese Patent Application Laid-Open (KOKAI) No. 54-144306 is advantageous in that the temperature of the bottom liquid in the distillation column can be restricted because water exists in the bottom liquid, it is disadvantageous in that since the distillation and separation in the low-boiling point component separation step is carried out by the indirect heating, there is a loss in the alkadienols and the palladium catalyst due to the local heating by the heating surface of the heat exchanger.
As a result of studies undertaken by the present inventors so as to solve the above-described problem, it has been found that in the process for producing alkadienols comprising the steps of: reacting a conjugated alkadiene with water in a reaction solvent in the presence of a palladium catalyst; distilling the obtained reaction mixture so as to separate the reaction solvent from the reaction mixture; and phase-separating bottoms discharged from the bottom of a distillation column, when the reaction mixture is distilled under the condition that the temperature of the bottom liquid in the distillation column is not higher than 120.degree. C. while directly supplying heated water and/or steam as a heating medium to the distillation column, the alkadienols is not decomposed nor polymerized and the palladium catalyst is not metallized, so that there is hardly any loss in the alkadienols and the palladium catalyst.